Hi Steve, I have built a few crates. Keep them light and cheap; 4-6mm ply, each side framed on the outside with 2×2cm wood (use glue and nails), screw and glue the sides together though the outside framing (except top use screws only) line the inside with polostyrene, if the framed images are not protected by glass wrap each frame in plastic to stop the image rubbing against another. place frames face-to-face : back-to-back.
hope this helps